This invention relates to a dimensional gage positioning system and, particularly, to one especially adapted for applying a gage device to a reciprocating engine crankshaft journal during a grinding process.
Recent advancements in the grinding of pin journals on internal combustion engine crankshafts have resulted in a shift away from traditional crank pin grinding.
Crankshafts have main bearing journals which define the axis of rotation of the crankshaft as it rotates in the engine, and further have a number of radially offset pin journals. Traditional grinding methods require that the crankshaft be positioned about the centerline of each individual pin journal during the grinding process. Refixturing of the crankshaft for phase angle, axial position, and radial offset is required for every pin journal. Now, with the capabilities of Computer Numerical Control (CNC) machine tools, the grinding process consists of fixturing the crankshaft only once on its main bearing centerline and rotating it just as it would rotate in the engine. All of the fixturing issues of the traditional method have been replaced by CNC programmable variables. The wheelslide of the grinder which mounts the grinding wheel moves dynamically to xe2x80x9cchasexe2x80x9d the pin journal currently being ground, while at the same time gradually advancing until an in-process diameter gage tells the machine that it has reached the desired final diameter.
To control this process, a gage must be capable of xe2x80x9cchasingxe2x80x9d the pin being ground while it rotates in a circular orbit. Since the gage itself is quite lightweight, it may be driven through its required motions by the crankpin journal itself if a suitable positioning system is provided. This positioning system must also function as an actuator, to advance the gage onto the pin journal and to retract the gage far enough to allow for part repositioning, and part unloading and loading. This mechanism would preferably provide positive control over the gage to prevent applying it mispositioned, which could result in xe2x80x9ccrashingxe2x80x9d with the CNC grinder or the workpiece and, therefore, damaging the gage.
The gage head typically used in crankshaft grinding processes consists of a gage frame designed to be mounted to a specialized gage support and an actuator. One end of the frame supports a xe2x80x9cveexe2x80x9d block whose function is to support replaceable wear pads within an included angle that, in turn, bear against the workpiece. The design of the gage and frame is such that the xe2x80x9cveexe2x80x9d contacts remain in contact with the workpiece at all times throughout the orbiting motion. As the grinding process decreases the size of the workpiece, the gaging xe2x80x9cveexe2x80x9d advances. This motion is directly and precisely monitored by means of an active probe contact located between the two wear pads of the xe2x80x9cveexe2x80x9d. This active contact is connected to a plunger that transfers the relative motions of the active contact with respect to the gage frame to a standard electronic pencil probe installed at the other end of the gage frame. This probe converts position information into an electrical signal that directly relates to the diameter change of the workpiece.
As stated above, the positioning system for the orbital gage preferably serves a dual function. First, it must advance and retract the gage to and from the workpiece. Second, the positioning system must act as a support for the gage during the orbiting motion of the workpiece. This support must have compliance in the plane of motion defined by the orbiting action of the workpiece, while at the same time, exhibit quite rigid support for the gage in all other degrees of freedom. Gage accuracy is directly dependent on these features of the positioning system.
The gage head positioning system of this invention is mounted on top of the grinder wheelslide assembly. This location is provided by the grinder manufacturer, as it simplifies the problem of removing the gage from the workpiece load/unload path. In addition, it greatly simplifies the motion that the positioning system must have during the actual grinding process. The motion of the workpiece, in the reference frame of the wheelslide, is an arc along the front surface of the grinding wheel. The gage moves vertically with a magnitude equal to the chord of this arc and horizontally with a magnitude equal to the rise of this arc.
The main functional component of the positioning system of this invention is the pivot arm assembly, having a lightweight pivot arm journaling pivot shafts at each end, with one point shaft mounted to the actuator base frame. The pivot arm assembly further includes a tierod also journaled to the actuator base. A link is affixed to the tierod and pivot arm by pivot shafts. The gage mounts to a gage mount arm coupled to the link with the gage frame xe2x80x9cveexe2x80x9d facing downward to straddle the workpiece. The gage is held in contact with the workpiece by gravity, and constrained to stay on the pin by the self-centering effect of the xe2x80x9cveexe2x80x9d.
The pivot arm, along with a tierod, the actuator base, and the link, form a four-bar linkage. This linkage assures that the gage remains in the correct orientation to xe2x80x9cfindxe2x80x9d the workpiece as it advances. Equally important, the gage is positively located when it is disengaged from the workpiece and cannot swing into contact with the grindwheel during the loading and unloading process. The geometric relationship of the four linkage elements allows the gage to be accurately located in the retracted position as well, close to, but not touching the actual elements of the wheelslide assembly.
The gage frame xe2x80x9cveexe2x80x9d sits on the workpiece angled away from the grind wheel in order to provide necessary wheel clearance. Because of this non-symmetrical orientation relative to the downward force of gravity, a prevailing torque is applied to the gage by the positioning system to optimize performance. This torque is provided by a spring-loaded pivot joint between the gage mount arm and the pivot arm link. Hard stops are also part of this pivot joint, to prevent the gage from exhibiting any more horizontal freedom of movement than that necessary to follow the workpiece orbit.
Design features are provided to keep the gagehead and moving portions of the positioning system light in weight to minimize the adverse effects of inertial loads between the gage and the workpiece. However, the contact force between the gage and the workpiece will vary greatly due to the vertical cycling of the mechanism. A counterspring assembly is provided within the actuator to reduce the magnitude of this cyclical loading. This assembly contains adjustments for spring position and spring rate. These adjustments allow the counterspring to provide appropriate characteristics for all workpiece sizes within the grinder""s capabilities.
Retraction of the gage is by means of a bellcrank mounted to the hub portion of the actuator pivot arm, and a hydraulic cylinder fixed to the actuator base. When the cylinder rod is extended, it meets the bellcrank, lifting the gage into the retracted position. When the cylinder rod is retracted, the gage is allowed to drop down onto the part. The cylinder rod continues to retract away from the bellcrank, becoming completely decoupled during gaging.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings.